This invention relates broadly to furnaces, boilers, incinerators, and like equipment wherein fuel is burned. More particularly, this invention relates to combustion equipment wherein combustion air is preheated. Still more particularly, this invention relates to combustion equipment wherein combustion air is preheated by a circulatable fluid transferring heat from one section of the equipment to the combustion air.
Equipment requiring the combustion of fuel constitutes a major class of equipment utilized in industry. Typically such equipment comprises settings which define combustion chambers furnished with one or more burners. Upon completion of the combustion process and delivery of the heat generated thereby to its intended sink, the flue gases pass to a stack from which they are vented to the atmosphere.
For equipment with a given fuel input, efficiency depends upon how much of the heat released from the fuel can be recovered. Stated differently, efficiency is an inverse function of flue gas temperature. One approach toward reducing stack temperature is to use the flue gas to preheat combustion air for the burners. This preheating may be accomplished by well known heat exchangers, wherein for example the combustion air is passed on the tube side of a shell and tube exchanger and the flue gas is passed on the shell side (or vice versa) for non-contact heat exchange one with the other. It is also well known to preheat air in regenerative heat exchangers wherein a heat storage mass is contacted alternately with the flue gas for heat collection and then with the combustion air for heat donation. Preheating of combustion air yields high efficiencies and has the added advantage of reducing fuel costs since it becomes unnecessary to heat the combustion air from ambient temperature up to the operating combustion temperature of the unit.
Unfortunately, there must be superimposed on the consideration of efficiency at least the added test of economic justification wherein a compromise must be reached between initial cost and operating cost. It is frequently possible to justify greater initial costs by reducing operating costs, but each equipment installation must usually stand on its own merits.
The economic feasibility of regenerative or indirect type air preheaters is commonly limited to very large equipment installations. One explanation of this limitation is that the cost of this type of preheat equipment does not go down with size as rapidly as basic equipment costs. Economic feasibility of this type of preheat equipment is further aggravated by the problems incident to sealing and moving substantial quantities of gases through large ductwork with fans, as well as the employment of substantial plant area for such ductwork, equipment, and fans. The regenerative type systems have the added burden of additional rotating equipment, usually subject to substantial corrosion, involving considerable maintenance to drives, motors, seals, and other moving parts.
An alternative to the regenerative or indirect type preheat system is the closed loop employing a heat transfer fluid circulated in a non-contact heat exchange relationship first with the flue gas for heat collection and then to the combustion air for heat donation. The inherent difficulties in the closed loop system include the requirement of expansion and surge tanks, inert blanketing systems, maintenance of a supply of inert purging medium, and storage facilities for the heat transfer fluid, together with complex triggering and control systems therefor, to activate an immediate and complete purging of the loop in the event of malfunction of the circulating pump. Metals must be used in the heat collecting coil which will withstand the maximum temperature of flue gas contacting the coil to avoid damage to the coil during those times when the coil is purged and the equipment is still operating.
It is also well known to preheat combustion air utilizing a portion of a process fluid stream entering the equipment for heating wherein the auxiliary stream is divided from the entering process stream and circulated in non-contact heat exchange relationship with combustion air by means of an air preheat coil and then subsequently cycled through a convention economizer coil for collection of heat from effluent flue gases from the equipment before recombining with the process stream either at the process stream's point of entrance into or exit from the equipment.
The inherent disadvantage with the auxiliary stream or slip stream, method of accomplishing the preheating of combustion air lodges in the feature of that system that limits the minimum inlet temperature to the heat donation coil and the minimum outlet temperature from the heat collecting coil to the temperature of the process stream from which the auxiliary stream was divided. It is well known that the driving force for heat transfer is the temperature difference between the fluid which is being heated and the fluid from which that heat is derived. The rate at which the heat flows from one fluid to another increases with that temperature difference, and conversely, the amount of heat absorbing surface, hence the cost thereof, changes inversely with that temperature difference.
In the normal heat exchange relationship, the rate and quantity of heat recovery by convection from flue gases are a function of three temperature differences, viz., flue gas entrance temperature versus flue gas exit temperature; heat collecting fluid entrance temperature versus flue gas exit temperature; and heat collecting fluid exit temperature versus flue gas entrance temperature. The designer, in contemplating an air preheat application, is compelled by economics and the overall heat balance within the system dictated by the physical properties of air and flue gas, to achieve a relatively low flue gas exit temperature from the primary heating service that the equipment is designed for. Or stated another way, relative to a process furnace, the designer must achieve a relatively low differential between the exit flue gas temperature and the incoming process stream temperature.
Relating the foregoing with the outlet temperature from the heat collecting coil, which, in the slip or auxiliary stream concept, cannot be lower than the incoming process stream temperature, the designer is encumbered with an impractically low differential temperature, or driving force, between his heat collecting coil and the flue gas. This condition dictates a heat collecting coil that is physically and economically out of proportion to the quantity of heat available for collection.
Historically, the combustion equipment industry, in utilizing the extant combustion air preheat systems, has been burdened with a choice between different forms of overcomplexity, high maintenance, outsize, or inflexibility, and general application to only the larger combustion applications.